In the process of electrophotography, the light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with a subsequent rendering of that latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of paper, with subsequent affixing of the image thereto.
In order to fix or fuse electroscopic toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner become tacky. This action causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification occurs causing the toner material to become bonded firmly to the support member. In electrophotography, the use of thermal energy for fixing toner images onto a support member is old and well known.
One approach to thermal fusing of electroscopic toner images has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of this type of fusing system, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll, thereby heating the toner image within the nip. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner at which temperature the toner particles in the image areas of the toner liquify and cause a splitting action in the molten toner resulting in "hot offset." Splitting occurs when the cohesive forces holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contacting surface, such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e., "cold" offsetting). This is generally caused by imperfections in the properties of the surface of the roll, or by the toner particles not adhering to the copy sheet as a result of insufficient adhesion forces. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the back-up roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or back-up roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus. The presence of such wayward toner particles can result in poor copy quality.
Most fusers of the type described above employ some method of applying a release fluid to the hot roll. Because of their inherent temperature resistance and release properties, silicone oils are typically used to prevent toner from adhering to the surface of the fuser roll and thereby degrading image quality and contaminating the fuser surface. The silicone oil also extends the life of the fuser rollers by providing some measure of lubrication to reduce the wear caused by the cumulative action of tens of thousands of pages passing through the pressure nip of the fuser. In order to assure the positive effects of the release fluid, a minimum amount of oil (typically 10-100 micrograms per page) is required.
Since the oil used as a release agent is partially carried away by the paper passing through the fuser system, it is necessary to ensure that the amount of oil dispensed is not so much that objectionable print quality defects are seen. In extreme cases, the surface of the imaged page can become visibly wet or glossy with oil. In cases of duplex printing (i.e., printing on both sides of the page), a more subtle effect is seen. In that instance, oil is carried back through the printing process by duplexed pages and the oil on those pages is deposited on various machine surfaces, including the photoconductor. It has been found that minute amounts of oil, invisible to the eye, can be enough to drastically effect the transfer of toner from the developer roll to the photoconductor. Since the development process depends upon a scrubbing action between the toned developer and the imaged photoconductor to aid in the transfer of toner from the developer to the photoconductor, and since the scrubbing action is induced by a mismatch in surface speed between the developer and the photoconductor, the addition of silicone oil at the interface of the two surfaces reduces the frictional scrubbing force to a level where transfer of toner can be severely impaired. Such print quality defects are very apparent in fine resolution printing. In extreme cases, the lack of toner transfer is seen even in 12 point text as light print. Levels of oil exceeding 100 micrograms per page can cause severe print defects if the distribution of oil across the page is not uniform. Typical print quality defects are white streaks in gray scale, with the streaks parallel to the process direction in areas of high oil concentration. For this reason, the upper limit of oil metering is about 100 micrograms per page.
A typical lubricant metering scheme employed in low-cost desk top printers involves saturating a felt pad constructed of temperature resistant material (such as DuPont's NOMEX Fiber) with silicone oil of a viscosity such that, when combined with the fiber size and density of the felt, the rate of flow out of the felt can be controlled within reasonable limits. Typical construction of a wiper pad includes application of an amount of silicone oil (e.g., 7-8 grams) of viscosity about 30,000 cst (at room temperature) to a precut piece of felt (e.g., fiber diameter 9 microns, felt density.perspectiveto.55 oz./yd.sup.2.), and baking the felt/oil combination at a high temperature to allow the oil to soak into the felt. Before high resolution printing (1200 DPI) and duplex printing, such a scheme was an excellent metering system, controlling oil flow within a range of 50-500 micrograms per page, with reasonable flow uniformity and no image defects. When this metering scheme is used with 1200 DPI and duplex printing, however, the previously acceptable non-uniform distribution of oil produces oil concentrations in some areas that are high enough to result in the above-described print defects.
Another requirement of the oil application system is that the amount of oil dispensed must be consistent during the life of the applicator (typically about 14,000 pages). As previously mentioned, failure to maintain adequate oil flow causes toner to adhere to the fuser and reduces the life of the fuser. Also, low flow allows toner to be collected on the felt applicator; when enough toner accumulates, a mass of toner breaks free and adheres to the page causing another print quality defect called "wiper dump." The felt applicator is a gravity feed system. This means that oil flows out of the felt at a constant rate for a given temperature. Silicone oil continuously flows out of the wiper even if the printer is not printing and is at standby. Thus, if the printer is at standby for a sufficient amount of time, the first page printed will receive an abnormally large amount of silicone oil and show duplex streaks.
In summary, for optimum performance the release agent oil application system must meet the following requirements:
Sufficient and consistent oil flow over the life of the system to prevent adherence of toner to the fuser roll. This extends fuser life and prevents wiper dumps. Minimum flow rate of 10 micrograms per page. PA1 A maximum flow rate of 100 micrograms per page and uniform flow to prevent image defects when printing high resolution images at 1200 DPI and in duplex mode. PA1 High thermal stability. The wax can have no odor throughout the life of the composition and should not appreciably change physical properties, such as viscosity. PA1 The wax must have a melt viscosity of from about 2,000 to about 10,000 centipoise, preferably from about 3,000 to about 7,000 centipoise, most preferably about 3,500 cps, at about 93.degree. C. This matches the silicone oil viscosity at fusing temperature and allows the wax to be directly substituted into the felt pad dispensing system. PA1 The wax must have a melting point between about 45.degree. C. and about 80.degree. C. If the melting point is below about 45.degree. C., the wax will not solidify when the printer is running at full speed and the cartridge is hot; thus, duplex streaks can occur. If the melting point is above 80.degree. C., the wax will solidify on the backup roll when printing heavy media and collect paper dust and toner which could cause the media to wrap the backup roll. PA1 At standard flow rates, the wax must not produce streaks on transparencies. This is accomplished by having a flow rate of less than about 800 micrograms per page.
U.S. Pat. No. 4,185,140, Strella, et al., issued Jan. 22, 1980, describes polymeric release agents for use on hot fuser rolls in an electrophotographic duplication process. The polymer materials utilized must include functional groups such as carboxy, hydroxy, isocyanate, thioether or mercapto groups. These materials are said to form a thermally stable release layer on the fuser roll which has excellent toner release properties. It is taught that the polymer material may be solid at room temperature, as long as it is a liquid at the temperature of the fuser. The materials disclosed as release agents are not silicone oils or waxes.
The present invention defines release agents which, when used on a hot fuser roll in an electrophotographic process, eliminate the problems described above. The material is liquid on the fusing surface and solidifies on the print medium when cool. It serves as an effective release agent (i.e., it prevents toner from adhering to the surface of the fuser roll and lubricates the fuser roll) and prevents print quality defects which result from the presence of oil on the paper, particularly during duplex printing.
Print quality issues can also arise as the result of toner leaks from the print cartridge. The toner cartridge is a replaceable supply used in printers and photocopiers. Its function is to hold a supply of the toner in a reservoir and then transfer the toner from that reservoir onto the developer roll, where it is present as a monolayer. The toner is then transferred onto the photoconductor in a pattern corresponding to the image to be printed, based on a charge distribution created on the photoconductor surface. Toner cartridges are well-known in the electrophotographic art and are, for example, described in U.S. patent application Ser. No. 08/770,330, Coffey, et al., filed Dec. 20, 1996, U.S. Pat. No. 5,802,432 incorporated herein by reference.
Careful transfer of the toner is critical to obtaining good printed images. Leaking toner will result in poor image quality, as well as soiling of the user's hands, clothing and office. A particularly troublesome spot on the cartridge, where toner leakage is likely to occur, is at the ends of the developer roll. In fact, toner cartridges frequently utilize specific seals, such as the J-seal, to prevent toner leakage at the ends of the developer roll. Despite that, some leakage still occurs due to variability in cartridge parts and assembly. A liquid or grease seal could be considered for use in the cartridge, for example at the ends of the developer roll. However, that could cause problems since such materials tend to migrate and migration of the sealant into the print area can cause contamination of the developer roll, the photoconductor and the charge roll, thereby causing print defects.
Therefore, it is a further object of the present invention to provide a silicone copolymer which, when formulated to have a paste or caulk-like consistency, serves as an effective sealant for use on toner cartridges, which does not migrate, is easy to apply and can be spread as a thin layer.